The present invention is directed to a four-stroke internal combustion piston engine with at least two axially extending cylinders each located on an opposite side of a crankshaft and extending axailly outwardly from the crankshaft. A piston is located within each cylinder and is arranged to reciprocate in the axial direction between the inner and outer ends of the cylinder with the pistons arranged to operate with a 180.degree. phase shift. Each piston has a piston rod connecting it to the crankshaft. Each piston has a bottom dead center adjacent the inner end of the cylinder closer to the crankshaft and a top dead center adjacent the outer end of the cylinder more remote from the crankshaft. A suction system is associated with the cylinders for supplying one of fresh air or a fuel-air mixture into the inner cylinder space and a transfer system for transferring the fresh air or fuel-air mixture from the inner space to an outer cylinder space on the opposite side of the piston. When the piston moves toward the outer end it draws the fresh air or fuel-air mixture into the inner cylinder space while the other piston moves toward the inner end of the other cylinder and displaces the fresh air or fuel-air mixture through the transfer system for flow into the outer cylinder space forming a combustion chamber.
There are a number of different internal combustion piston engines with the cylinders located opposite one another and extending outwardly on the opposite sides away from a crankshaft housing. When the pistons move outwardly from the crankshaft housing they draw fresh air or fuel-air mixture into the interior of the crankshaft housing and then compress the fresh air or fuel-air mixture as the piston moves inwardly and at the same time direct the air or fuel-air mixture into a cylinder through an overflow or transfer channel.
Therefore, in the German Offenlegungsshrift No. 33 15 853 a process for operating a four-stroke motor with cylinders disposed in pairs extending opposite one another is disclosed with the pistons operating with a 180.degree. phase shift. The pistons move in opposite directions and their reciprocating motion is converted into rotational motion by a crankshaft drive. During the simultaneous movement of both of the pistons in the outward direction, that is, toward the cylinder head, fresh air or a fuel-air mixture is drawn through an inlet check valve into the crankcase. Subsequently, during the simultaneous inward stroke, that is, as both pistons move toward the crankshaft, the air previously drawn in, is pressed out of the crankcase through a branch channel of a common overflow or transfer channel system into the combustion space of only one of the two oppositely disposed cylinders, that is, in the cylinder which executes the intake stroke while the power stroke is effected in the other cylinder. During the next crankshaft turn the air or fuel-air mixture is drawn into the crankshaft housing through the previously mentioned intake check valve by the action of the two outwardly moving pistons and then the air or fuel-air mixture is transferred into the other cylinder as the two pistons move inwardly.
This known process has disadvantages in that the charging or boosting quality for the individual cylinders is insufficient, particularly in high speed engines. This occurs as a result of the intake stroke when the two pistons are moving outwardly, the fresh air or the air-fuel mixture for a respective cylinder must initially be accelerated or moved in its entire mass by the two pistons, since before that time no noticeable compression could occur because the intake valve for the cylinder to be charged had already opened during the outward movement of the two pistons. Due to the inertia of the intake gas column which must be accelerated, certain charging losses occur which reduce the specific output of the cylinder or the so-called volumetric output. Furthermore, the fresh air or fuel-air mixture is conveyed through the interior of the crankshaft housing which is also disadvantageous.